Combined touchscreen and membrane switch

ABSTRACT

A combined membrane switch and touchscreen employs a continuous front transparent sheet that directly supports a transparent electrode for the touchscreen and graphics for the membrane switch portion of the assembly. Improved transparency and reduced touch activation pressure is thereby obtained.

CROSS-REFERENCE TO RELATED APPLICATIONS STATEMENT REGARDING FEDERALLYSPONSORED RESEARCH OR DEVELOPMENT BACKGROUND OF THE INVENTION

[0001] The present invention relates to electrical membrane switchesand, in particular, to a combination of one or more membrane switchesand a touchscreen such as may fit over a graphic display terminal.

[0002] Membrane switches provide front and rear sheets held inseparation by a spacer layer. In one common version, electrical contactsare printed with conductive inks on the opposed surfaces of the frontand rear sheets. The conductive inks may for example be those includingmetallic silver inks applied by a printing process such as silkscreening.

[0003] The front sheet of the membrane switch is flexible so that it maybe deformed inward, by pressure of a fingertip or the like, to touch therear sheet through apertures in the spacer layer. When the sheets touch,the electrical contacts on their surfaces allow the flow of electricalcurrent. A desirable property of membrane switches is that they presenta continuous front surface resistant to contamination from moisture anddirt.

[0004] In a well known variation on the above design, the conductor onthe front sheet is replaced with “a snap dome” being conductive orhaving a conductor printed on its surface facing the rear sheet.Pressure on the front sheet may cause the snap dome to snap from itsconvex configuration with its rear surface removed from the rear sheetto a concave mode in which its rear surface and conductor shortsconductive fingers on the rear sheet, again allowing current flow.

[0005] A single front and rear sheet may provide for a number ofseparately actuable switches or button by printing separate conductorsand forming multiple apertures in the spacer layer. Normally the frontlayer is printed with button symbols denoting the location of theseswitches and where pressure should be applied to activate each switch.The front layer is typically transparent with button symbols printed onthe rear surface to protect them from abrasion.

[0006] In contrast to a membrane switch, a resistive touchscreenprovides an indication of a touch point arbitrarily located on the faceof a CRT or other electronic display screen. Like a membrane switch, aresistive touchscreen employs front and rear sheets with conductivecoating material, but in a touchscreen the conductive coating must be atransparent material such as ITO (Indium Tin Oxide) to allow viewing ofthe terminal on which they are overlaid. In a resistive touchscreen (incontrast to a matrix touchscreen which works like a membrane switch),the conductive materials are laid continuously over the front and rearsurface rather than in discrete locations. An ITO coating is relativelyexpensive and so the area of the touchscreen is ordinarily sized closelyto the area of the display screen to minimize the amount of ITOrequired.

[0007] There are many kinds of resistive touchscreens (Three-wire,Four-wire, Five-wire, Six-wire, Seven-wire, and Eight-wire resistivetouchscreens). All of these resistive touchscreens measure a voltagegradient produced as current flows through the transparent resistivecoating. The Three-wire, Five-wire, Six-wire, and Seven-wiretouchscreens utilize a slightly different detection algorithm than theFour-wire and Eight-wire touchscreens.

[0008] With Five-wire resistive touchscreen, a DC voltage, for example5V, is first applied vertically to the rear sheet of uniform conductivematerial (first, second, third and fourth wires). This will create avoltage gradient from top to bottom. When the front sheet is touched,the conductive material of the front sheet, which connects to controllerelectronics (via the fifth wire), probes the voltage at the touch pointof rear sheet. The percentage of probed voltage over 5V will be thepercentage of the distance of the touch point over the height of thetouchscreen. For example, if probed voltage is 2.5V, that means themiddle of the height of the touch screen has been touched. Once thevertical location is determined, the DC 5V is switched to be appliedhorizontally to the conductive material of rear sheet (first, second,third and fourth wires). The probed voltage at touching point will thentell the horizontal location of the touch point.

[0009] For Four-wire resistive touchscreens, the vertical location isdetected just like with the Five-wire touchscreen with the gradientapplied via the first and second wires and using one or both of thirdand fourth wires on the front sheet to probe the voltage. However thehorizontal location is detected by applying the dc voltage to the frontsheet using the third and fourth wires and using the rear sheetconductive to probe the voltage via one or both of the first and secondwires.

[0010] For the purpose of accurately locating the touch point, it isdesirable that the conductive material on the touchscreen has a veryuniform resistive distribution in front and rear sheets for Four-wireresistive touchscreen and very uniform resistive distribution in rearsheet for five-wire resistive touchscreen. In contrast, the conductorsused in the membrane switch are desirably those having the lowestpractical resistance only. In following disclosure, resistive touchreferring to this invention includes all different types of resistivetouchscreens as described above.

[0011] Membrane switches and a touchscreen may be used together, forexample, on an input terminal where membrane switches flank the displayscreen. The membrane switches may provide fixed location functions andthe touchscreen may provide functions based on the output of the displayscreen. One method of combining membrane switches and a touchscreen forapplications like this is to cut a window in the center of a membraneswitch panel through which the touchscreen is exposed. This approachcreates sealing difficulties that may allow contaminates to enterthrough the cut out area and may therefore defeat one of the desirableproperties of such switches. An alternative method of combining membraneswitches and a touchscreen is to mount the touchscreen under the toplayer of the membrane switch. This top layer which normally supportsgraphics for the membrane switch is left clear in the area of thetouchscreen. Although this approach eliminates the problems with a seam,the graphics layer of the membrane switch over the touchscreensignificantly increases the touch activation force for the touchscreenand reduces the optical transmission of light from the display screen.

[0012] Desirably a combined membrane switch and touchscreen could beproduced without these drawbacks.

BRIEF SUMMARY OF THE INVENTION

[0013] The present invention provides a combined touchscreen andmembrane switch having a single continuous front layer that is shared bythe membrane switch as its graphics layer and the touchscreen assupporting its conductive material. Use of a single layer for these twopurposes improves optical transmission through the touchscreen andreduces the amount of force needed to activate the touchscreen.

[0014] The membrane switches may also use this top layer for support oftheir electrodes or may include a second electrode support layer. Thislatter option increases the force of activation slightly for themembrane switch, but this is acceptable for membrane switch operation.The rear transparent layer of the touchscreen may also be extended toprovide a portion of the membrane switch circuitry.

[0015] Specifically then, the present invention provides a combinedtouchscreen and membrane switch sharing a front layer formed of acontinuous flexible sheet with a transparent portion supporting atransparent conductor on a rear surface, and a graphic portion printedwith button symbols. A spacer layer is positioned behind the front layerand has openings aligned with the transparent portion and at least onebutton symbol. A transparent rear layer is positioned behind the spacerlayer to support a transparent conductor on a front surface opposed tothe transparent portion of the front layer. Membrane switch circuitry ispositioned behind the graphic portion of the front layer and alignedwith the button symbols to be activated upon pressing of the buttonsymbols.

[0016] It is one object of the invention to provide a combinedtouchscreen/membrane switch presenting a seamless front surface andtouch activation force for the touchscreen comparable with stand alonetouchscreens.

[0017] The graphic portion of the front layer may also support aconductor on its rear surface.

[0018] Thus, it is another object of the invention to use the frontlayer as an electrode support both for the membrane switch andtouchscreen simplifying the design and further reducing the forcerequired to activate the touchscreen.

[0019] The conductor on the rear surface of the graphic portion of thefront layer may be of a material different from the conductor on therear surface of the transparent portion of the front layer.

[0020] Thus, it is another object of the invention to optimizeconductors for touchscreens and membrane switches on a single layer.

[0021] The conductor on the rear surface of the transparent portion ofthe front layer may be a conductive polymer.

[0022] Thus it is yet another object of the invention to provide atransparent conductor that may be patterned locally on a continuousfront layer to significantly reduce the cost of using a single frontlayer for both a membrane switch and a touchscreen.

[0023] The transparent rear layer may extend behind both the transparentportion and the graphics portion of the front layer and the membraneswitch circuitry may include a conductor supported on the front surfaceof the rear layer behind the graphic portion of the front layer.

[0024] Thus, it is another object of the invention to provide a simpledesign in which a single layer supports circuitry for both the membraneswitches and the touchscreen.

[0025] Many of the objects and advantages described above apply to onlysome embodiments falling within the claims and thus do not define thescope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026]FIG. 1 is an exploded perspective view of the touchscreen/membraneswitch assembly of the present invention as positioned in front of adisplay screen showing transparent portions and graphic portions of theassembly;

[0027]FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1showing a first embodiment in which opposed continuous front and rearlayers hold conductors of both the membrane switch and touchscreen;

[0028]FIG. 3 is a figure similar to that of FIG. 2 showing a secondembodiment employing a supplemental electrode support layer for themembrane switch; and

[0029]FIG. 4 is a figure similar to that of FIG. 2 showing an embodimentfor producing a touchscreen with graphic margins using the techniques ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0030] Referring now to FIG. 1, a combined touchscreen/membrane switch10 provides graphic portions 12 having printed button symbols 14 and atransparent portion 16 through which the faceplate 18 of a graphicdisplay 20 such as a cathode ray tube may be viewed.

[0031] In use, the touchscreen/membrane switch 10 allows a user toprovide electrical inputs by pressing the button symbols 14 or bytouching arbitrary areas 21 of the transparent portion 16 correspondingto images 22 on the faceplate 18 of the graphic display 20.

[0032] Referring now to FIG. 2, the combined touchscreen/membrane switch10 provides a front layer 24 formed in part of a continuous, flexibleand transparent sheet 26, for example, a heat stabilized polyester film.In the transparent portion 16, a rear surface of the transparent sheet26 is coated with a transparent conductor 28 being, for example, indiumtin oxide, or in the preferred embodiment, printed with a conductivepolymer such as ORGACON, based on a polyethylene dioxithiophene andavailable from Agfa-Gevaert N. V. of Mortsel, Belgium. The transparentconductor 28 is generally evenly applied over the entire area of thetransparent portion 16 commensurate with the area of the faceplate 18.

[0033] The conductive polymer is less costly than ITO thus allowing itto be applied to the entire surface of the front layer 24. Further, theconductive polymer may be printed to be applied to the transparent sheet26 only where necessary. This is in contrast to the ITO which generallymust be applied to the entire surface of the transparent sheet 26 andthen etched away from regions where it is not desired.

[0034] Referring still to FIG. 2, the graphic portions 12 of front layer24 has at its rear surface printed graphics 30 such as depict buttonsymbols 14 or an opaque matrix surrounding them. The graphics 30 areprinted on the rear surface of the transparent sheet 26 protecting themfrom abrasion. 100371 Also applied to the rear surface of transparentsheet 26 either behind the graphics 30 (as shown) or in portions wherethe graphics 30 are not placed, and thus directly attached to thetransparent sheet 26, are low resistance conductors 32. Such conductorsmay, for example, be screen printed silver or carbon and are intended topresent low resistance conductors in contrast to the higher more easilymeasured resistance of the material of the transparent conductor 28. Thelow resistance conductors 32 provide contact circuitry for the membraneswitches as is understood in the art.

[0035] Positioned behind the front layer 24 is a spacer layer 36comprised, for example, of an insulating plastic sheet 38 having a frontand rear adhesive coating 40 and 42, respectively. The spacer layer 36presents apertures 44 aligned with the button symbols 14 shown in FIG. 1and contact pads of the low resistance conductors 32 of FIG. 2. Thespacer layer 36 also presents an aperture 46 equal in area to thetransparent portion 16 exposing the faceplate 18 of the graphic display20. Optionally the spacer layer 36 also includes insulating dots 48positioned in the transparent portion 16 as understood in the art anddescribed above.

[0036] The spacer layer 36 separates the front layer 24 from a rearlayer 50. Like front layer 24, rear layer 50 is formed in part of acontinuous, flexible, and transparent sheet 52, for example, a heatstabilized polyester film. Alternatively, the rear layer may be a rigidtransparent sheet such as plastic or glass or a rigid sheet in thetransparent portion and a flexible sheet in the graphics portion 12.

[0037] In the transparent portion 16, the front surface of transparentsheet 52 is coated with the conductive polymer to provide a transparentconductor 54 opposing transparent conductor 28 of the front layer 24. Arear surface of the transparent sheet 52 supports an adhesive 56allowing it to be adhered to the faceplate 18 of the graphic display 20.

[0038] At graphic portion 12, the transparent sheet 52 includes on itsfront surface, a low resistance conductor 58 being comparable to lowresistance conductors 32 of the front layer 24 and printed, for example,of a silver or carbon ink. The low resistance conductor 32 providescontact circuitry working with contact circuitry formed by lowerresistance conductors 32 on the front layer and also can provideinterconnection circuitry for the membrane switches and the touchscreen, connecting these elements to leads attached at the rear layer 50of the touchscreen/membrane switch 10. The rear surface of thetransparent sheet 52 of the graphic portion also provides adhesive 56for attaching the graphics portion of the touchscreen/membrane switch 10to a supporting substrate (not shown).

[0039] It will be understood that this embodiment provides an extremelysimple construction. In alternative embodiments, the front layer 24 iscontinuous but the spacer layer 36 and the rear layer 50 need not becontinuous but may be separate elements as desired. In addition, therear layer 50 need not be a flexible material but may include, forexample, printed circuit board material, glass, or the like.

[0040] Referring now to FIG. 3, in a second embodiment, a supplementalelectrode support layer 62 is added under the graphic portion 12 of thefront layer 24 and in front of the spacer layer 36. The supplementalelectrode support layer 62 is formed of a flexible material 66 having onits front surface an adhesive 68 to attach it to the rear surface of thefront layer 24.

[0041] Generally, the supplemental electrode support layer 62 eliminatesthe need to place low resistance conductors 32 directly on graphics 30as described above with respect to FIG. 2. The front layer 24 in thetransparent portion 16, as before, is formed of transparent sheet 26 andtransparent conductor 28. The front layer 24 in the graphic portion 12,however, supports only graphics 30 and does not support a low resistanceconductor 32. Instead, the electrode support layer 62 supports lowresistance conductors 32 on its rear surface standing in lieu of thefront layer 24. Activation of the membrane switch must now deform boththe front layer 24 and the supplemental electrode support layer 62;however, greater touch forces are more easily accommodated with thenormal operation of a membrane switch.

[0042] The embodiment of FIG. 3 employs a rear layer 50 similar to thatwhich has been described above.

[0043] In an alternative embodiment (not shown), the supplementalelectrode support layer 62 may be replaced with a snap dome as is wellknown in the art.

[0044] Referring now to FIG. 3, the spacer layer 36 may incorporate oneor more conductors 70 providing a path of conduction from thetransparent conductor 28 on the front surface to electrical pathsformed, for example, by low resistance conductor 58 on the rear layer 50allowing external connections to the touchscreen/membrane switch 10 tobe made exclusively at the rear layer 50 for manufacturing convenience.The conductor 70 may, for example, be a conductive epoxy, conductivetape or an anisotropically conductive paste for example 3M Z-AxisAdhesive paste commercially available from 3M Corporation of Minnesotasuch as allows for electrical conduction between front layer 24 and rearlayer 50 with relatively little lateral or in-plane conduction.

[0045] Referring now to FIG. 4, the continuous front layer 24 providedby the present invention may also be used to produce a graphics margin70 to the side of the transparent portion 16 in which the touchscreen isimplemented. In this case, the transparent portion 16 is constructed asdescribed above with respect to FIGS. 2 and 3. The graphic margin 70uses the same transparent sheet 26 of the front layer 24 as is used inthe transparent portion 16 but without the rear layer of transparentconductor 28. As was mentioned above, the use of a printable transparentconductor 28 makes this practical. The graphic margin 70 is not limitedto regions around the edges of the transparent portion 16 but may alsoinclude small graphic regions positioned within the transparent portion16 if so desired.

[0046] In place of the transparent conductor 29, in the graphics margin,the transparent sheet 26 has on its rear surface printed graphics 30similar to those described above with respect to FIG. 2 but not ofbutton symbols. In this embodiment, the graphics margin 70 does notinclude any membrane switch circuitry and is followed simply by a spacerlayer 36 and a rear layer 50 having no switch conductors. The spacerlayer 36 and the rear layer 50 may be otherwise constructed as describedabove. The graphic margin may be decorative or may provide instructionsor the like to the user of the touchscreen. Again, the benefits of acontinuous front layer 24 are obtained.

[0047] It is specifically intended that the present invention not belimited to the embodiments and illustrations contained herein, butinclude modified forms of those embodiments including portions of theembodiments and combinations of elements of different embodiments ascome within the scope of the following claims.

I claim:
 1. A combined touchscreen and membrane switch comprising: a) afront layer formed of a continuous flexible sheet and having: i) atransparent portion supporting a transparent conductor on a rearsurface; and ii) a graphic portion printed with button symbols; b) aspacer layer positioned behind the front layer and having openingsaligned with the transparent portion and at least one button symbol; c)a transparent rear layer positioned behind the spacer layer supporting atransparent conductor on a front surface opposed to the transparentportion of the front layer; and d) a membrane switch circuitrypositioned behind the graphic portion and aligned with the buttonsymbols to be activated upon pressing of the button symbols; wherein asingle surface is presented to the user having minimum resistance totouchscreen operation.
 2. The combined touchscreen and membrane switchrecited in claim 1 wherein the rear layer is rigid transparent sheetselected from the group consisting of glass and transparent plastic. 3.The combined touchscreen and membrane switch recited in claim 1 whereinthe rear layer is flexible sheet facing graphic portion and rigid sheetfacing transparent opening for touchscreen
 4. The combined touchscreenand membrane switch recited in claim 1 wherein the graphic portion ofthe front layer also supports a conductor on a rear surface.
 5. Thecombined touchscreen and membrane switch recited in claim 4 wherein theconductor on the rear surface of the graphics portion is of a materialdifferent from the conductor on the rear surface of the transparentportion of the front layer.
 6. The combined touchscreen and membraneswitch recited in claim 4 wherein the conductor on the rear surface ofthe graphic portion of the front layer is silver ink.
 7. The combinedtouchscreen and membrane switch recited in claim 4 wherein the conductoron the rear surface of the transparent portion of the front layer is aconductive polymer.
 8. The combined touchscreen and membrane switchrecited in claim 4 wherein the conductor on the rear surface of thetransparent portion of the front layer is an indium tin oxide.
 9. Thecombined touchscreen and membrane switch recited in claim 1 includingfurther a supplemental electrode support layer positioned between thegraphics portion of the front layer and the spacer layer, wherein thesupplemental electrode support layer supports a conductor on a rearsurface.
 10. The combined touchscreen and membrane switch recited inclaim 8 wherein the conductor on the rear surface of the supplementalelectrode support layer is silver ink.
 11. The combined touchscreen andmembrane switch recited in claim 1 wherein the transparent rear layerextends behind both the transparent portion and the graphics portion ofthe front layer and wherein the membrane switch circuitry includes aconductor supported on the front surface of the rear layer behind thegraphics portion of the front layer.
 12. The combined touchscreen andmembrane switch recited in claim 11 wherein the conductor supported onthe front surface of the rear layer behind the graphics portion are of amaterial different from the transparent conductor on the front surfaceof the rear layer behind the transparent portion of the front layer. 13.The combined touchscreen and membrane switch recited in claim 12 whereinthe conductor on the front surface of the rear layer behind the graphicsportion is silver ink.
 14. The combined touchscreen and membrane switchrecited in claim 13 wherein the conductor on the front surface of therear layer behind the transparent portion of the front layer is aconductive polymer.
 16. The combined touchscreen and membrane switchrecited in claim 13 wherein the conductor on the rear surface of thetransparent portion of the front layer is an indium tin oxide.
 17. Thecombined touchscreen and membrane switch recited in claim 11 includingfurther a supplemental electrode support layer positioned between thegraphics portion of the front layer and the spacer layer, wherein thesupplemental electrode support layer supports a conductor on a rearsurface.
 18. The combined touchscreen and membrane switch recited inclaim 17 wherein the conductor on the rear surface of the supplementalelectrode support layer is silver ink.
 19. The combined touchscreen andmembrane switch recited in claim 1 wherein the spacer layer provides aconductive path between the front layer and the rear layer allowingeternal electrical connections to be made solely at the rear layer. 20.A touchscreen with graphic margin comprising: a) a front layer formed ofa continuous flexible sheet and having: i) a transparent portionsupporting a transparent conductor on a rear surface; and ii) a graphicportion proximate to the transparent portion printed with graphics; b) aspacer layer positioned behind the front layer and having at least oneopening aligned with the transparent portion; c) a transparent rearlayer positioned behind the spacer layer supporting a transparentconductor on a front surface opposed to the transparent portion of thefront layer; and wherein a single surface is presented to the userhaving minimum resistance to touchscreen operation.